1. Field of the Invention
The present invention relates to laser scanning systems for reading and decoding bar code symbols. Particularly, it relates to an object sensing circuit that (i) detects when an object has been introduced into the scan field of the laser scanner and (ii) automatically converts the laser scanning system from a sleep or object sensing mode of operation to a continuous or scan mode of operation when an object has been detected.
2. Discussion of Prior Art
Various optical readers and optical scanning systems have been developed heretofore for reading bar code symbols appearing on a label or on the surface of an article. The bar code symbol itself is a coded pattern of indicia comprised of a series of bars of various widths spaced apart from one another to bound spaces of various widths, the bars and spaces having different light reflecting characteristics. The readers and scanning systems electro-optically transform the graphic indicia into electrical signals, which are decoded into alphanumerical characters that are intended to be descriptive of the article or some characteristic thereof. Such characters are typically represented in digital form and utilized as an input to a data processing system for applications in point-of-sale processing, inventory control, and the like. Scanning systems of this general type have been disclosed, for example, in U.S. Pat. Nos. 4,251,798, 4,369,361, 4,387,297, 4,409,470, 4,760,248, and 4,896,026, all of which have been assigned to the same assignee as the instant application.
Bar code symbols are formed from bars or elements that are typically rectangular in shape with a variety of possible widths. The specific arrangement of elements defines the character represented according to a set of rules and definitions specified by the code or "symbology" used. The relative size of the bars and spaces is determined by the type of coding used, as is the actual size of the bars and spaces. The number of characters per inch represented by the bar code symbol is referred to as the density of the symbol. To encode a desired sequence of characters, a collection of element arrangements are concatenated together to form the complete bar code symbol, with each character of the message being represented by its own corresponding group of elements. In some symbologies a unique "start" and "stop" character is used to indicate where the bar code begins and ends. A number of different bar code symbologies exist. These symbologies include UPC/EAN, Code 39, Code 128, PDF417 Codabar, and Interleaved 2 of 5.
For the purpose of our discussion, characters recognized and defined by a symbology shall be referred to as legitimate characters, while characters not recognized and defined by that symbology are referred to as illegitimate characters. Thus, an arrangement of elements not decodable by a given symbology corresponds to an illegitimate character(s) for that symbology.
In order to increase the amount of data that can be represented or stored on a given amount of surface area, several new bar code symbologies have recently been developed. One of these new code standards, Code 49, introduces a "two-dimensional" concept by stacking rows of characters vertically instead of extending the bars horizontally. That is, there are several rows of bar and space patterns, instead of only one row. The structure of Code 49 is described in U.S. Pat. No. 4,794,239, which is hereby incorporated by reference.
A one-dimensional single-line scan, as ordinarily provided by hand-held readers, has disadvantages in reading these two dimensional bar codes; that is, the reader must be aimed at each row, individually. Likewise, the multiple-scan-line readers produce a number of scan lines at an angle to one another so these are not suitable for recognizing a Code 49 type of two-dimensional symbols.
In the scanning systems known in the art, the light beams are directed by a lens or similar optical components along a light path toward a target that includes a bar code symbol on the surface. The scanner functions by repetitively scanning the light beam in a line or series of lines across the symbol. The scanning component may either sweep the beam spot across the symbol and trace a scan line across and past the symbol, or scan the field of view of the scanner, or do both.
Scanning systems also include a sensor or photodetector which functions to detect light reflected from the symbol. The photodetector is therefore positioned in the scanner or in an optical path in which it has a field of view which extends across and slightly past the symbol A portion of the reflected light which is reflected off the symbol is detected and converted into an electrical signal, and electronic circuitry or software decodes the electrical signal into a digital representation of the data represented by the symbol that has been scanned. For example, the analog electrical signal from the photodetector may typically be converted into a pulse width modulated digital signal, with the widths corresponding to the physical widths of the bars and spaces.
Such a signal is then decoded according to the specific symbology into a binary representation of the data encoded in the symbol, and to the alphanumeric characters so represented.
Laser scanners are not the only type of optical instrument capable of reading bar code symbols. Another type of bar code reader is one which incorporates detectors based upon charge coupled device (CCD) technology. In such readers size of the detector is larger than or substantially the same as the symbol to be read. The entire symbol is flooded with light from the reader, and each CCD cell is sequentially read out to determine the presence of a bar or a space. Such readers are lightweight and easy to use, but require substantially direct contact or placement of the reader on the symbol to enable the symbol to properly read. Such physical contact of the reader with the symbol is a preferred mode of operation for some applications, or as a matter of personal preference by the user.
There presently exists various types of fixed mount omnidirectional laser scanners for bar code reading applications. These scanners typically use one or more motor/mirror assemblies to generate a scan pattern. The light source used to scan bar codes is usually a semiconductor visible laser diode. For a typical omnidirectional laser scanner, the scan pattern is designed such that the bar code may be scanned in any orientation in a plane with respect to the scan pattern. This obviates the need for the user of the scanner to carefully align the bar code symbol with respect to the scan pattern as one would have to do for single line laser scanners, whether fixed mount or hand held.
Fixed mount omnidirectional scanners are often operated in a continuous ON mode. In this mode, items having bar codes may be rapidly scanned with no need for the user to carefully align the bar code symbol nor to initiate the scanning process by turning on the scanner in any way. A key disadvantage of continuous mode operation is that the key internal components of the laser scanner, such as the visible laser diode and the scan motor, have limited operational lifetimes and their degradation is accelerated by continuous operation. For instance, the reflective facets which define the laser cavity tend to degrade in reflectivity over time resulting in the gradual reduction of effective laser output power. Also, the various motors used as scanning elements all comprise mechanical parts having finite lifetimes. Other factors that may degrade motor lifetime are high speeds, unbalanced motor shaft loads, and high temperatures associated with constant on or high duty-cycle operation.
Because of the limited lifetime associated with the key elements of the laser scanner, it is undesirable to have the scanner remain on during extended periods of non-use. Thus, to extend the overall scanner lifetime, the scanner may be shut down if it is not used for a predetermined length of time. The shutting down process may be implemented in hardware or software. In prior art laser scanners, this shutting off process may have required the user to manually restart the scanner, possibly by toggling an external switch on the scanner or controller. This is not desirable because time is wasted during the power-up process and especially because that process is not transparent to the user.
In view of the above-mentioned disadvantages associated with an omni-directional laser scanner having only a continuous ON and OFF mode of operation, a system that provides a way for the laser scanner to operate in a dormant state or "sleep" mode of operation, when not in use, would be highly desirable. Such a sleep mode would permit the laser scanner components such as the visible laser diode to remain OFF, and the motor(s) to operate at a fraction of the nominal scanning speed, thus helping to reduce overall power consumption and significantly extend the lifetimes of these components. Furthermore, a system that can rapidly convert from a sleep mode to a continuous ON mode of operation in a way that is virtually transparent to the user would also be advantageous. It is accordingly an object of the present invention to fulfill these needs by providing an object sensing system that automatically converts the laser scanner from a sleep mode to a continuous mode of operation when an object is introduced in the scanning field. Such an object sensing system provides circuitry that detects when an object is placed in the scanning field, such as when a user attempts to read a bar-coded item. Once detected, the object sensing system automatically "wakes up" to scan and decode the symbol. Thus the present invention enables the user of bar code scanners to conserve the useful lifetime of the critical scanner components including the laser diode and scan motors without sacrificing the ease of use or scanner throughput.